The present invention relates to wind turbines and more particularly to a wind turbine of the type which is fabricated with a rotor operable about a horizontal axis and including a flexible beam member and blades supported at the opposite ends thereof and which is adapted to yaw about a vertical axis.
Efforts to reduce dependence upon fossil fuels has led to increasing demand for efficient and relatively rugged wind turbines which will provide substantial electric power in areas where the prevailing winds are sufficient to effect operation of the wind turbines over substantial periods of time. The wind turbines which are achieving most widespread utilization at the present time utilize a horizontal axis for rotation of the rotor and provide some means to permit or to drive the nacelle or power unit to rotate around a vertical axis so as to permit the assembly to yaw into a downwind position for optimum operation. In permitting or causing the yawing of the assembly to take place, there is a tendency to cause the power cables and control cables within the supporting structure to twist and this may, if unrestrained, cause these cables ultimately to fail. In an effort to eliminate this problem, slip rings have been utilized in some turbine structures at the top of the tower to permit rotation of the elements of the assembly without causing twisting of the wires. Such slip ring assemblies are fairly costly to install and require substantial maintenance. In still other assemblies, separate drive and control means have been used to vary the yaw angle of the rotor assembly with high initial cost and maintenance factors.
In many of the modern wind turbines, it is common to employ a flexible beam to the ends of which the rotor blades are secured and to allow the blades to twist about their longitudinal axis so that a change in pitch can be effected as the rotational speed increases. In an effort to provide relatively light weight rotors and the desired flexibility of the rotor blades to permit torsional forces to effect twisting of the blades and thereby pitch control, fiber reinforced synthetic resins are widely employed in the fabrication of the flexible beam and of the blades. Because of the flexural characteristics of these rotors, there is a tendency for the blades to cone or bend in the direction of the supporting tower in the event that the assembly is in the upwind position or in the event that wind forces are sufficiently gusty so as to cause flapping or bending of the blades out of the plane of rotation and towards the tower. If the blades were to strike the tower, this could result in severe damage to the wind turbine itself.
A further problem that is encountered with downwind turbines is that, when the blades are disposed at an angle to the vertical, there is low or negative static stability in an upwind position so that the rotor cannot rotate about its vertical axis to the necessary downwind position. It is therefore desirable to provide means for rotating the blades into a vertical position so that they will produce positive static stability and readily commence yaw rotation to the downwind position for startup. To provide means for effecting such orientation of the blades, various control mechanisms and independent drive mechanisms have been suggested to rotate the blades into the vertical position. Obviously such control mechanisms and independent drive mechanisms to effect the vertical orientation of the blades in the static stability condition involve added cost and maintenance factors.
In some wind turbines, there has been provided a pendulum mechanism to effect twisting of the flexible rotor blades in response to increasing rotational speed in order to maintain the blade pitch angle at approximately 0.degree. throughout the designed operating range of the wind turbine. Such apparatus is disclosed in Applicant's U.S. Pat. Nos. 4,352,629 granted Oct. 5, 1982, and 4,083,651 granted Apr. 11, 1978. In using the type of mechanism described therein, problems have been experienced with respect to creep or permanent set induced in the synthetic plastic flexible beams which are customarily employed, which set alters the design characteristics of the rotor and results in less efficiency of the system. Moreover, additional problems have been encountered due to the high forces exerted on the pivot point for the pendulum arm causing frequent repair and replacement of all or portions of the assembly.
It is an object of the present invention to provide a novel wind turbine having improved operating mechanisms to minimize the aforesaid problems.
It is a specific object to provide such a wind turbine in which there is provided improved pitch control means for the blade, which means is relatively rugged and long-lived in operation and which will minimize the tendency for the rotor structure to exhibit excessive creep.
Another specific object of the present invention is to provide such a wind turbine in which there is included simple and relatively rugged means for orienting the blades in an approximately vertical position during periods when the wind turbine is upwind as the wind commences and to avoid creating a negative static stability situation if the blades were in a horizontal position.
Still another specific object is to provide such a wind turbine wherein there is included simple, rugged and highly effective means for controlling the amount of yawing of the rotor head and reducing the tendency to produce excessive twisting of the cables and control wires extending therethrough.